This invention relates to a circuit for producing large pulses of electric current especially useful in welding.
Pulsed electric currents of high value are used in many applications, a particularly important one is in welding including spot welding. Other important applications include pulsed electroplating, D.C. to D.C. inverters, D.C. electric motor speed controls and surface hardening by pulsed arcs. In these applications an approximately square wave current pulse is desired and current pulses of up to 10,000 amp may be required. It is for such large current pulses that silicon control rectifiers ((SCR's) are most appropriate as solid state switching devices.
The preferred embodiment of the invention to be described uses SCR's as switching elements in its circuit which is used to control the output of a dc power supply of conventional nature and the one pulse unit can be used with a wide range of power supplies such as motor generator sets, lead-acid storage batteries and transformer-rectifier units. In this fashion the cost of installing pulsed welding facilities, for example, is significantly reduced because continuous current welding power supplies which have already been installed, can be used with the pulsing unit.
LAWRENCE, B. and JACKSON, C. in 1969 Variable Frequency Gas Shielded Pulse Current Arc Welding. Welding Journal Res. Suppl. 48 1969, 97s-104s. reported a study of pulsed current arc welding in which the influence of the pulse current frequency, duration and magnitude on the mode of metal transfer in the MIG (metal-inert gas) arc was determined. This experimental equipment used SCR's switching the output of a dc power supply with the control pulses to the gates of the SCR's being obtained from a mechanical commutator. In order to maintain a continuous standing current so that the arc did not go out at the end of the current pulse a second power supply was used. The use of a mechanical commutator and multiple power supplies would not now be seen as elegant design. Furthermore the switching circuit which used two SCR's functioned in a bistable mode of operation in which each SCR was used to turn the other off alternately. The larger SCR controlled the arc pulse current and the smaller SCR conducted for the time that the arc current pulse was not flowing. This method of operation is not only inefficient but also enhances the problem of disposal of surplus heat produced by the pulse generator.
Two other pulse shaping units for welding have been described which switch the output of a welding power supply, SMITH G. A. and BROWN, M. J. An Inverter Power Source for Welding Applications. 2nd International Conference on Power Electronics--Power Semiconductors and their Applications. London England: I.E.E. 1977 58-61, and LOWERY, J. A New Concept for AC/DC Power Sources for TIG-Welding. Advances in Welding Processes 4th International Conference Harrogate England, 4th-11th May 1978, 161-169. These designs have been developed specifically for TIG (tungsten-inert gas) welding of aluminium for which application it is highly advantageous to have the current through the arc alternating thus producing a cleaning action on the workpiece surface by ion bombardment when it is biased negative with respect to the tungsten electrode. Each pulse unit is of fixed frequency (50 or 60 Hz). The cleaning effect as well as the weld penetration and heat input to the workpiece are changed (but not independently) by varying the ratio of the durations of the positive and negative half cycles of current.
Another device described by GRIST, F. J., Improved, Lower Cost Aluminium Welding with Solid State Power Source. Welding Journal. 54 1975, 348-357, is generally similar to those mentioned immediately above but offers in addition limited control of the frequency of operation (50-200 Hz). This frequency limitation is imposed by the use of a resonant commutation circuit for the SCR's.
One of the shortcomings of the above and other known circuits is that the control of the switching of the current pulses may be dependent on parameters which do not reflect the conditions at the arc itself.
The object of the present invention is to provide a circuit for generating current pulses of up to at least 10,000 amps in which the switching of the current pulses is controlled in accordance with the voltage at the load.